Ultraviolet (UV) photodetectors and light emitters find numerous uses including applications in the defense, commercial, and scientific arenas. These include, for example, covert space-to-space communications, missile threat detection, chemical and biological threat detection and spectroscopy, UV environmental monitoring, and germicidal cleansing. Detectors and light emitters operating in the solar blind region are of special interest. The solar blind region corresponds to the spectral UV region where strong upper atmospheric absorption of solar radiation occurs, generally at wavelengths <290 nm. This creates a natural low background window for detection of man-made UV sources on and proximate to the earth's surface.
Semiconducting materials having a 25° C. band gap of about 4 to 6 eV have been used to sense or generate solar blind UV radiation. Conventional approaches have used semiconductors such as AlGaN, MgZnO, or BeZnO, which generally have wurtzite (hexagonal) lattice structures. AlGaN is known to suffer from various problems including cracking due to strain, generally high dislocation density, and lattice mismatch (all such effects are generally interrelated). High dislocation density undesirably reduces internal quantum efficiency. The use of wurtzite MgZnO is limited due to phase segregation that occurs for mid-Mg compositions as a result of a solid solubility limits and mixed phase regions. BeZnO is a somewhat more promising material, but has experienced doping difficulties, particularly difficulties in getting high mobility and stable p-type doping.
Additionally, problems arise from the lack of a suitable lattice matched substrate, leading to higher dislocation densities, and also the polarization fields that are common with these wurtzite (hexagonal) lattice structures. Accordingly, high quality single crystal epitaxial articles, and a process for forming the same for operation at wavelengths <290 nm, particularly in the 200-290 nm region, are needed.